1. Field of Invention
The present invention relates to hydrostatic transmissions intended primarily for use in the lawn and garden industry on tractors, riding lawnmowers, lawn and garden implements and the like.
2. Description of the Related Art
Hydrostatic transmissions transmit rotary mechanical motion, typically from an internal combustion engine, to fluid motion, typically via positive displacement pumps and motors using oil, and then back to rotary mechanical motion to rotate a drive axle in order to drive the vehicle. The hydrostatic transmission controls the output rotary mechanical motion such that varying output speeds in the forward and reverse directions are possible with a single speed input rotary mechanical motion. Such transmissions have utilized radial piston pumps and motors, axial piston pumps and motors and hybrid transmissions wherein the pump may be of one piston design, and the motor of another. The speed of the output of the transmission is typically controlled by varying the eccentricity of the pump track ring of a radial piston pump or the swash plate angle of an axial piston pump.
Hydrostatic transmissions have an inherent problem of not achieving, when placed in neutral, a condition in which the pump displacement is completely eliminated. Although the operator may shift the implement into neutral, thereby causing the hydrostatic transmission to be placed in neutral, there may still be some motion, or xe2x80x9ccreepxe2x80x9d, of the implement. During forward or reverse operation of the hydrostatic transmission, this fluid is constantly moving through the system. In neutral, ideally, the displacement of the rotating pump is zero, and no fluid flows to the motor therefrom. Thus, no motion, however slight, is imparted to the axle. Should the rotating pump still have some slight displacement in neutral, however, fluid in one side of the hydrostatic system will become or remain slightly pressurized and cause the motor to slowly rotate, thereby creating forward or reverse motion of the wheels. What would be desirable is a hydrostatic transmission which allows any fluid displaced by the pump to be vented out of the hydrostatic system when the hydrostatic transmission is placed in the neutral position, thereby eliminating creep.
Yet another problem associated with the use of hydrostatic transmissions is the xe2x80x9cjerkingxe2x80x9d effect created when the swash plate is moved from neutral to forward or reverse and vice versa. Dampening of the engagement or disengagement of the hydrostatic transmission would eliminate the jerking or at least xe2x80x9csoftenxe2x80x9d the transition to and from neutral. What would be desirable is a hydrostatic transmission which includes a mechanism for dampening the response of the motor to changes in pump displacement rates as the pump approaches and leaves neutral so that such jerking would be eliminated.
The present invention provides a hydrostatic transmission including a fluid motor, a variable displacement fluid pump in fluid communication with the fluid motor, a pump mounting block having an outer surface and provided with at least one fluid passage through which fluid from the pump flows to the motor, a fluid sump external to the block, a cylindrical bore disposed within the block and having at least two apertures located diametrically opposite each other, and a sliding element disposed within the cylindrical bore. The pump has a first fluid displacement rate and a second fluid displacement rate, which is much greater than the first displacement rate. The pump mounting block has at least one first fluid bleed hole extending from the fluid passage to one of the apertures in the cylindrical bore and at least one second fluid bleed hole extending from the other of the apertures in the bore to the outer surface of the block. The sliding element is provided with at least one void and has a first position in which the void is in fluid communication with the fluid passage through the first fluid bleed hole and the void is in fluid communication with the sump through the second fluid bleed hole, and a second position in which the void is substantially out of fluid communication with the fluid passage and the sump. The fluid passage and the sump are in fluid communication through the first bleed hole, the void, and the second fluid bleed hole when the pump is operating at its the first displacement rate and the sliding element is in its first position, and the fluid passage and the sump are substantially out of fluid communication when the pump is operating at its second displacement rate and the sliding element is in its said second position.
The present invention further provides a hydrostatic transmission including a fluid motor, a variable displacement fluid pump in fluid communication with the fluid motor and having a first fluid displacement rate and a second fluid displacement rate, which is much greater than the first displacement rate, a pump mounting block having a flat surface against which the pump is slidably engaged when the pump is operating at its first and second fluid displacement rates, a fluid sump external to the block, and means for placing the passage and the sump in fluid communication when the pump is operating at its first fluid displacement rate and providing a gradual motor response to changes between the pump first and second fluid displacement rates. The means for placing the passage and the sump in fluid communication further includes means for maintaining the passage and the sump substantially out of fluid communication when the pump is operating at its second displacement rate. The block is provided with at least one fluid passage which opens to the flat block surface, fluid which flows from the pump to the motor flowing through the passage.
The present invention also provides a method for dampening the response of a fluid motor to changes in a fluid pump between a neutral position and a drive position in a hydrostatic transmission, and preventing fluid from being pumped by the fluid pump to the motor in the pump neutral position, including rotating the pump while maintaining its sliding engagement against a block having at least one passage therethrough, operating the rotating pump at a first displacement rate in its neutral position gradually decreasing the fluid communication between the passage and the sump while changing from the pump neutral position to the pump drive position, operating the rotating pump at a second displacement rate greater than the first displacement rate when in its drive position, through the passage for driving the motor, and gradually increasing the fluid communication between the passage and the sump while changing the pump from its drive position to its neutral position. In the pump neutral position, the passage and a sump are in fluid communication, whereby fluid displaced by the pump in its neutral position is directed to the sump. In the pump drive position, the passage and the sump are substantially out of fluid communication, whereby fluid displaced by the pump in its drive position is directed to the motor.
An advantage provided by the present invention is that any fluid displaced by the pump in neutral is vented out of the hydrostatic system, thereby preventing the occurrence of creep in the forward or reverse direction.
An additional advantage provided by the present invention is that it dampens the effect of changes in pump displacement to and from zero displacement by allowing a portion of the hydrostatic fluid to bleed or be vented out of the hydrostatic system as the transmission is shifted from neutral to an operative condition in forward or reverse, and vice versa.